GarageBand: "Core Audio Plug-in EXS24... not available" error when loading a song

If you create a song in GarageBand on one computer and then try to open it in GarageBand on another computer, you may get an alert message that says, "Core Audio Plug-in EXS24... not available."

This can happen if you create the song in GarageBand 2 and then try to open it in GarageBand 1 on another computer, because GarageBand 2 includes new plug-ins that are not available in GarageBand 1. However, this can also happen if a song references Jam Pack plug-ins that are not installed on the computer on which you're currently working.

For greater freedom, consider upgrading all the computers you use to the same version of GarageBand and install the same Jam Packs.